The invention relates to a device for encoding digital signals corresponding to images subdivided into blocks, which device is composed of a first encoding channel comprising a discrete cosine transform circuit, a quantizing circuit having a determined first quantization step, a variable-length encoding circuit, a buffer memory supplying encoded signals having a determined quality level, and a rate control circuit, a prediction channel comprising, at the output of said quantizing circuit, a first inverse quantizing circuit, an inverse discrete cosine transform circuit, a first adder, a picture memory, a motion compensation stage, and a subtracter for subtracting the predicted output signals of said stage from said digital signals to be encoded, and a second encoding channel comprising a circuit for computing the differences on the basis of signals downstream of the discrete cosine transform circuit, a circuit for quantizing these differences with a second quantization step which is finer than the first step, and a circuit for encoding the differences thus quantized. The invention is particularly applicable in the field of distributing television pictures having two image quality levels and is compatible with the MPEG standard.
For transmitting images in a digital channel, it is necessary to compress the information contained in these images, while taking the rate of the existing channels into account. For this purpose there are numerous encoding techniques and one of the most currently used techniques is a successive mathematical transform referred to as discrete cosine transform (DCT), followed by a quantization of coefficients obtained by said transform and a variable-length encoding of the values thus quantized, which operations are completed by a prediction of motion of images. The temporal prediction is performed on the basis of said quantized values and, upon encoding, does not present the signals which correspond to each current image but signals which are representative of the differences between this current image and the preceding image, while taking the motion between these two images in the time interval separating them into account.
A device of this type is described in U.S. Pat. No. 4,958,226. Its structure, including initially the first encoding channel and the prediction channel for obtaining a first image quality level, also comprises a second channel for encoding a value referred to as residual error and taken from said first encoding channel. The image which is subsequently decoded and restored thus benefits from complementary information by which a second image quality level can be obtained.